Radio positioning systems

ABSTRACT

The present invention sets out to overcome the hearability problem in CDMA communications networks in which positioning services are provided, by using a separate sampling device ( 204, 205, 206 ) for each transmitter ( 201, 202, 203 ), which sends to the computing device ( 208 ) a representation of the signals transmitted only by that transmitter. A cross-correlation of the representation sent back by the mobile terminal ( 207 ) with the representation sent back by the sampling device in the brightest transmitter is performed in the computing device ( 208 ), and an estimate of that brightest signal is subtracted from the representation sent back by the mobile terminal ( 207 ) in order to reduce its effect on the remaining signals as far as possible. The cross-correlation and subtraction steps are iterated until no useful signals remain to be extracted.

The present invention relates to radio positioning systems generally,and more particularly to improved methods of finding the positions ofmobile terminals in radio communication systems, especially thoseemploying Code Division Multiple Access (CDMA) technology.

There are many systems known by which the position of a mobile terminaloperating in a radio communications network may be determined. Theseinclude using the signals from transmitters not connected with thenetwork, such as the Global Positioning System (GPS) satellites, butothers make use of the signals radiated by the mobile terminal andpicked up by remote receivers, such as the Time Of Arrival (TOA) andso-called “Radio Finger Printing” systems or, vice versa, using thesignals radiated by the network itself and picked up by the mobileterminal. Chief amongst the last category are the Enhanced Observed TimeDifference (E-OTD) and Observed Time Difference Of Arrival (OTDOA)systems.

The E-OTD system, although generally applicable to many differentcommunication technologies, has been particularly applied to the GlobalSystem for Mobiles (GSM). Two principal, and different, methods of usingthe timing offsets of signals received from the network transmitters inthe position computation have been described in the art. In one, e.g.EP-A-0767594, WO-A-9730360 and AU-B-716647, the signals measured by afixed receiver are used, in effect, to ‘synchronise’ the transmissionsfrom the different transmitters. The instantaneous transmission timeoffsets of each transmitter relative to its neighbours are calculatedfrom the values measured at the fixed receiver using the known positionsof the fixed receiver and the transmitters. The timing offsets measuredby the mobile terminal can then be used in a calculation based onwell-known standard techniques in which the points of intersection oftwo or more hyperbolic position lines predict the position of the mobileterminal.

The other method (see our EP-B-0303371, U.S. Pat. No. 6,094,168 andEP-A-1025453 the details of which are hereby incorporated by referenceand which refer to a system known as Cursor™) makes use of themeasurements made by both the fixed receiver and the mobile terminal tocalculate the relative time difference between the signals received fromeach transmitter by both receivers. This results in a calculation basedon the intersection of circles centred on the transmitters.

E-OTD methods, as applied to GSM, have been considered for use inwide-band CDMA systems, in particular those within the Universal MobileTelephone System (UMTS) ‘third generation’ (3G) technologies. Here,E-OTD has been re-named OTDOA, but it suffers from a major problem, theso-called ‘hearability’ problem. In CDMA networks generally, signals aretransmitted by the network transmitters all using the same radiofrequency (RF) channel. In UMTS this channel is about 5 MHz wide. Thesignals from each transmitter are encoded using a unique ‘spreadingcode’ which allows a mobile terminal to pick out the required signalprovided that (a) it knows the spreading code used by that transmitter,and (b) its internal clock is synchronized with the transmitter signals.To assist with the latter, each transmitter also radiates a ‘pilot code’within the same RF channel whose coding and other characteristics makeit easily distinguishable. The mobile terminal first detects and lockson to the pilot signal, receives the spreading code used by thattransmitter, and then is able to decode the main transmissions. Thehearability problem arises when the mobile terminal is near to atransmitter. E-OTD systems (and therefore OTDOA systems) require themeasurements of the time offsets associated with at least threegeographically-distinct transmitters, but when the mobile terminal istoo close to a transmitter, the signals from the more-distanttransmitters are drowned out by the local signals to the extent thattheir time offsets cannot be measured. One technique, known as ‘IdlePeriod Down Link’ (IPDL), has been proposed to overcome this problem bywhich the transmissions from the local transmitter are turned offperiodically in a so-called ‘idle period’ during which the signals fromthe distant transmitters may be received. This has the seriousdisadvantages that (a) the capacity of the network to carry voice & datatraffic is diminished, and (b) it is complicated to install and operate,requiring in one of its forms additional messaging in the network tocoordinate the idle periods amongst the transmitters.

The present invention involves an adaptation of the Cursor™ system,especially as described in our U.S. Pat. No. 6,094,168, to CDMA systemsin general and particularly to UMTS in such a fashion as to overcome thehearability problem. No idle period is required, and the communicationsfunction can therefore operate with full capacity. It has the furtheradvantages that (a) the fixed receivers associated with E-OTD and OTDOAare particularly simple and low-cost devices, and (b) the additionalsoftware required in the mobile terminals is less complex than that inGSM terminals.

The Cursor™ system, as described in U.S. Pat. No. 6,094,168, uses tworeceivers, one fixed and at a known location and the other within themobile terminal, to receive the signals radiated by each transmittertaken separately. Representations of the received signals are sent backto a computing node where they are compared (generally bycross-correlation) to determine the time offset of receipt of thesignals by each receiver. This process is repeated for at least twoother geographically distinct transmitters (transmitting on different RFchannels in a GSM system) to obtain the three time offsets required fora successful position computation.

In direct sequence CDMA systems the transmitters use the same RFchannel. A direct application of the Cursor™ system to CDMA wouldtherefore result in a cross-correlation with many peaks, eachcorresponding to the alignment of the signals received from a particularone of the transmitters by both receivers. If it were possible tomeasure the peaks associated with at least the three requiredtransmitters, the system would serve admirably for positioning. However,as illustrated in the particular embodiment described below, the signalto noise ratios (SNRs) associated with more-distant transmitters areoften too small, and we have a similar hearability problem as describedabove.

A first aspect of the invention therefore provides a method of findingthe position or state of motion of a terminal in a communicationsnetwork having a plurality of transmitters, the terminal having a radioreceiver, the method comprising the steps of

-   -   (a) creating at the terminal a section of a representation of        the signals from the transmitters received by the radio        receiver;    -   (b) creating a first section of a representation of the signal        transmitted by a first of said transmitters, and creating a        second section of a representation of the signals transmitted by        a second of said transmitters, each of which sections overlaps        in time with the section created at the terminal;    -   (c) calculating an estimate of the signal received at the        terminal from said first transmitter using said first section,        and subtracting said estimate from the section created at the        terminal, to produce a residual representation;    -   (d) performing a calculation using said residual representation        and said second section, and estimating the time offset between        them; and    -   (e) calculating the position of the terminal using said time        offset.

Preferably, the first and second sections are created at the respectivefirst and second transmitters, but they may be created elsewhere. Theymay be created in one or more sampling devices attached to therespective transmitters or located elsewhere, or they may be created bycomputer programs running anywhere in the communications network, orelsewhere, using information supplied from the network about thetransmitted signals.

The various signal representation sections may be sent to one or morecomputing devices in which said estimates and time offsets, and theterminal location, may be calculated. In some embodiments, the timeoffset between said section of a representation of the signals receivedby the receiver and said first section may first be calculated, and maythen be used in the calculation of said estimate. The time offset may becalculated using said sections or it may be calculated by other means,for example by calculating the time offset of a known component of thesignal such as the pilot code.

The present invention thus overcomes the hearability problem by, forexample, using a separate sampling device for each transmitter, theequivalent of the fixed receiver in an E-OTD system, which sends to acomputing device a representation of the signals transmitted only bythat transmitter, by performing a cross-correlation of therepresentation sent back by the mobile terminal with the representationsent back by the sampling device associated with one of the transmittersto estimate the time offset between them, and by subtracting an estimateof that signal from the representation sent back by the mobile terminalin order to reduce its effect on the remaining signals as far aspossible. The cross-correlation and subtraction steps may be iterateduntil no useful signals remain to be extracted. Simulations show thatthis provides at least as much hearability gain as the IPDL method.However, of course, the transmitted signals are unaffected by the methodof the invention, so that, for example, the transmissions do not need tobe interrupted.

In some systems, the hearability problem may be solved simply bysubtracting an estimate of just one of the signals, usually thebrightest, leaving a residual representation in which the time offsetsof the pilot codes, or any other known portions of the transmittedsignals, may be determined.

Thus, a second aspect of the invention provides a method of finding theposition or state of motion of a terminal in a communications networkhaving a plurality of transmitters, the terminal having a radioreceiver, the method comprising the steps of

-   -   (a) creating at the terminal a section of a representation of        the signals from the transmitters received by the radio        receiver;    -   (b) creating a section of a representation of the signal        transmitted by one of said transmitters, which overlaps in time        with the section created at the terminal;    -   (c) calculating an estimate of the signal received at the        terminal from said transmitter using said section of the        representation of the signal transmitted by said transmitter,        and subtracting said estimate from the section created at the        terminal, to produce a residual representation;    -   (d) performing one or more calculations using said residual        representation and one or more known components of the signals        transmitted by the communications network in order to estimate        the time offset of the respective component; and    -   (e) calculating the position of the terminal using any of said        time offsets.

The section of the representation of the signals received by thereceiver at the terminal may be recorded in the terminal before beingsent to a computing device. Alternatively, the section may betransferred in real time to the computing device and a recording madethere.

Preferably, the section of the representation of the signals transmittedby a transmitter is created at said transmitter, but it may be createdelsewhere. It may be created in a sampling device attached to saidtransmitter or located elsewhere, or it may be created by a computerprogram running anywhere in the communications network, or elsewhere,using information supplied from the network about the transmittedsignals.

The calculations may be carried out in a computing device which may bein the handset or elsewhere, for example, a processor connected to thenetwork.

The representation of the signals received by the receiver attached tothe terminal may be a digitised version of the received signalsconverted first to baseband in the receiver. The representation of thesignals transmitted by a transmitter may be a digitised version of thetransmitted signals converted first to baseband.

In order to ensure an overlap of the respective sections, a suitablychosen component of the transmitted signals may be used to indicate thestart of sampling.

The time offset between said section of the signals received from atransmitter by the receiver attached to the terminal and said section ofa representation of the signal transmitted by one of said transmittersmay be computed using a cross-correlation or other comparison betweenthe respective sections, or it may be computed as part of the normalcommunications process in the terminal, or it may be computed using aknown component of the signals transmitted by the communicationsnetwork, for example a pilot code.

The known components of the transmitted signals in the second aspect ofthe invention may, for example, be the pilot codes.

The invention also includes apparatus for carrying out the invention.

Thus, there is provided, for use in the carrying out the method of thefirst aspect of the invention, a communications network, the networkcomprising

-   -   (a) a computing device or devices;    -   (b) a terminal having a radio receiver attached to the terminal,        means for creating a section of a representation of the signals,        received by the radio receiver, from the transmitters of the        communications network, and means for sending the section to the        computing device or devices;    -   (c) sampling devices associated with respective first and second        of said transmitters for creating respective first and second        sections of representations of the signal transmitted by the        respective transmitter which overlap in time with the section        created at the terminal, and for sending the sections of the        representations created at said transmitters to said computing        device or devices;    -   the computing device or devices being adapted to perform    -   1. a calculation of an estimate of the signals received at the        terminal from said first transmitter using said first section;    -   2. a subtraction of said estimate from the section sent by the        terminal, to produce a residual representation;    -   2. a calculation using said residual representation and said        second section to produce an estimate of the time offset between        them; and    -   3. a calculation of the position of the terminal using said time        offset.

The invention also includes a computing device or devices for use insuch a communications network, adapted to perform the tasks set out inthe paragraph immediately above.

The invention also includes, for use in the carrying out the method ofthe second aspect of the invention, a communications network comprising

-   -   (a) a computing device or devices;    -   (b) a terminal having a radio receiver attached to the terminal,        means for creating a section of a representation of the signals,        received by the radio receiver, from the transmitters of the        communications network, and means for sending the section to a        computing device;    -   (c) devices associated with the transmitters for creating        sections of representations, of the signal transmitted by the        respective transmitter, which overlap in time with the section        created at the terminal, and for sending said sections to the        computing device or devices;        the computing device or devices being adapted to perform    -   1. a generation of a reference signal;    -   2. a calculation of an estimate of the signal received at the        terminal from said transmitter using said section of a        representation of the signal transmitted by the corresponding        transmitter;    -   3. a subtraction of said estimate from the section sent by the        terminal to produce a residual representation;    -   4. one or more calculations using said residual representation        and said reference to estimate the time offset between the at        least one signal and said reference; and    -   5. a calculation of the position of the terminal using the or        any of the time offsets.

The invention also includes a computing device or devices for use insuch a communications network, adapted to perform the tasks set out inthe paragraph immediately above.

The means for carrying out the calculations in the computing device ordevices may be components of hardware and/or software.

Therefore, the invention includes a computer program or programs havingcomputer program code means for carrying out the steps performed in thecomputing device or devices as described above.

The terminal may be a part of a positioning system, for example asdescribed in any of EP-A-0767594, WO-A-9730360, AU-B-716647EP-B-0303371, U.S. Pat. No. 6,094,168 and EP-A-1025453 and may be afixed device associated with a transmitter (for example, the ‘fixedreceiver’ or ‘Location Measurement Unit, LMU’), whose purpose is toreceive signals from distant transmitters as well as from its associatedtransmitter, in which case the method of the invention includes theestimation of and subtraction of the signals from its associatedtransmitter in order to allow it to measure the time offsets of thesignals received from distant transmitters.

A third aspect of the invention therefore provides a method of findingthe time offset between at least one of the signals received from aplurality of the transmitters of a communications network by a receiverattached to a fixed terminal and a reference generated in the fixedterminal, the method comprising the steps of

-   -   a) creating at the fixed terminal a section of a representation        of the signals from the transmitters received by the radio        receiver;    -   b) creating a section of a representation of the signal        transmitted by one of said transmitters, which section overlaps        in time with the section created at the fixed terminal;    -   c) calculating an estimate of the signal received at the fixed        terminal from said transmitter using said section of a        representation of the signal transmitted by said transmitter,        and subtracting said estimate from the section created at the        fixed terminal, to produce a residual representation;    -   d) performing a calculation using said residual representation        and said reference in order to estimate the time offset between        a component of the residual representation and said reference.

The E-OTD positioning systems described generally above work withunsynchronised networks, i.e. any common component of the signalstransmitted by any one transmitter is not synchronised in time with thetransmission of that component by any other of the transmitters, butinstead is transmitted after an unknown time delay, sometimes called theRelative Transmission Delay (RTD). The position calculation requiresthat this delay is known, and so the positioning systems employ fixedreceivers at known locations throughout the network which are set up tomeasure the transmitted signals and compute the RTDs. It has beendescribed above how the hearability problem hinders the straightforwardapplication of the E-OTD techniques to direct-sequence CDMA systems.However, the third aspect of the present invention overcomes thehearability problem by allowing the very strong signals from a localtransmitter to be subtracted from the signals received by the fixedreceiver, thus allowing the much weaker signals from the distanttransmitters to be measured. The method of application of E-OTD to CDMAsystems then follows that described, for example, in our EP-A-1025453.

The invention may be further understood by reference to the accompanyingdrawings, in which:

FIG. 1 shows the geometry of a two-dimensional communications system inwhich all the transmitters and the mobile terminal lie in one plane;

FIG. 2 shows a simplified UMTS network;

FIG. 3 illustrates the correlation of a reference copy of the primaryscrambling code used on the pilot code channel (CPICH) by each Node B ofthe UMTS network with a recording of the received signal;

FIG. 4 shows the result of cross-correlating the recording received by aterminal with recordings of the transmitted signals;

FIG. 5 illustrates measured and estimated recordings;

FIG. 6 show the cross-correlation of a residual recording withrecordings of transmitted signals; and

FIG. 7 shows the cross-correlation of a further residual recording witha recording of a transmitted signal.

The following mathematical analysis provides an understanding of theconcepts involved in the present invention. FIG. 1 shows the geometry ofa two-dimensional system in which all the transmitters and the mobileterminal lie in one plane. The positions of transmitters A, B, and C arerepresented by the vectors a, b, c, all with respect to the same commonorigin, O. The mobile terminal, R, is at vector position r. Each of thetransmitters has incorporated with it a sampling device, as describedabove, which samples the signals transmitted by that transmitter andwhich sends back to a computing device (not shown in FIG. 1) arepresentation thereof. Let us suppose that the mobile terminal isnearest to transmitter A, then B, then C. The computing device firstperforms a cross-correlation between the representation of the signalsreceived from A, B, and C (all on the same RF channel) by R, and therepresentation of the signals transmitted by A. Since the signals fromA, B, and C have orthogonal spreading codes, the cross-correlationresults in a single peak whose position represents the time-offset ofthe receipt of the signals from A by R, together with the clock error,ε, of the receiver in the mobile terminal. This time offset, Δt_(A), isgiven byvΔt _(A) =|r−a|+ε,where v is the speed of the radio waves, and the vertical bars denotethe magnitude of the contained vector quantity. Similarly, for B and Cwe havevΔt _(B) =|r−b|+ε,vΔt _(C) =|r−c|+ε.  {1}

Having established the time offset of the signals from A, the computingnode now subtracts an estimate of the signal received from A by R. Therepresentations of the signals radiated at time t by the transmitters A,B, and C, may be denoted by the functions S_(A)(t), S_(B)(t), andS_(C)(t) respectively. The signal received by the mobile terminalcomprises a combination of these. In the absence of multipath, noise andnon-linear effects, the representation of the received signals may bedenoted by V(t), whereV(t)=αS _(A)(t−Δt _(A))+βS _(B)(t−Δt _(B))+γS _(C)(t−Δt _(C)).and α, β, γare constants representing the path losses to the mobileterminal from the respective transmitters. A software program running inthe computing node estimates the magnitude of S_(A)(t), delayed byΔt_(A), to subtract from V(t), for example by finding the value of awhich minimises the mean square amplitude of the residual V′(t). In theperfect case this would remove the contribution of A altogether, so thatV′(t)=βS _(B)(t−Δt _(B))+γS _(C)(t−Δt _(C)).

The cross-correlation is now carried out between V′(t) and S_(B)(t) toestimate Δt_(B), and a further subtraction made to remove thecontribution of B from the residual, V″(t), whereV″(t)=γS _(C)(t−Δt _(C)),if the subtraction is perfect. Finally, a cross-correlation betweenV″(t) and S_(C)(t) results in an estimate of Δt_(C). Equations {1} canthen be solved for r as described in U.S. Pat. No. 6,094,168.

In practice, the signals received by the mobile terminal are corruptedby noise, interference and multipath effects. Furthermore, therepresentations of the signals may be in a digital format of lowresolution. The process of subtraction will not be perfect in thesecircumstances, but may nevertheless be sufficient to overcome thehearability problem. Where it is possible to estimate the channelparameters, the effects of multipath propagation can be allowed for,resulting in better signal subtractions.

One of the requirements of the invention is that the recordings of thesignals made at A, B, C, and R overlap in time with each other. Therecording process in the mobile terminal can be initiated, for example,by the receipt of a particular aspect of the signal transmitted by theserving transmitter (A in the above analysis). The recordings made inthe transmitters must all be loosely synchronised with this aspect.Where the transmitters are synchronised with each other, as in the IS 95standard, the aspect will be transmitted at approximately the same timeby all transmitters in the network. In unsynchronised systems, however,other means such as GPS or the concepts described in our WO-A-00/73814and EP application no. 01301679.5 may be used for synchronisation.

One embodiment of a system according to the invention will now bedescribed with reference to FIGS. 2 to 7.

FIG. 2 shows a simplified UMTS system consisting of three communicationstransmitters (Node Bs) 201, 202, 203, each of which has a samplingdevice 204, 205, 206, a single terminal (user equipment, UE) 207, and acomputing device (serving mobile location centre, SMLC) 208. Each Node Bhas an omnidirectional antenna, and is configured to transmit signalstypical of network traffic load. Table 1 below indicates the differentphysical channels in use, together with their power levels and symbolrates. The acronyms appearing in the left-hand column, P-CPICH etc., arethose that have been adopted by the industry to represent the channels.Random binary sequences are used to modulate the DPCHs. The three NodeBs use orthogonal primary scrambling codes, in this case numbers 0, 16and 32 respectively.

TABLE 1 Node B channel configuration Channel Relative power Level/dBSymbol rate/Kss⁻¹ P-CPICH −10 15 P-SCH −10 15 S-SCH −10 15 P-CCPCH −1015 PICH −15 15 DPCH0 Note 1 Note 2 DPCH1 Note 1 Note 2 DPCH2 Note 1 Note2 . . . Note 1 Note 2 . . . Note 1 Note 2 DPCH63 Note 1 Note 2 DPCH64Note 1 Note 2 Note 1: DPCH power levels were chosen randomly from −10 dBto −25 dB Note 2: DPCH symbol rates were chosen randomly from 15 to 240Kss⁻¹

The Node Bs are tightly synchronised. This is not a requirement innormal practice, but is convenient for the purpose of demonstration.

It will be noted from FIG. 2 that the UE 207 is relatively close to NodeB 201 and at greater distances from Node Bs 202 and 203. Thus the signalfrom Node B 201 is the strongest (0 dB relative to itself) with thesignal from Node B 202 weaker at −15 dB and that from Node B 203 weakestof all at −30 dB. The three sampling devices 204, 205, 206 areinstructed by the SMLC 208 to record and report the signal transmittedby the associated Node B during the first 256 chips immediatelyfollowing the start of the next second. These signals are sampled at arate of 2 samples per chip, with a resolution of 4 bits.

Before describing how the system is used to illustrate the presentinvention, the problem of hearability is highlighted by considering theconventional E-OTD approach to measuring the time offsets of the signalsreceived by the UE 207. A reference copy of the primary scrambling codeused on the CPICH by each Node B (i.e. the first 256 chips of each ofscrambling codes 0, 16 and 32), is cross-correlated with the signalreceived by the UE 207 and a search is made for the highest correlationpeak. FIG. 3 illustrates a typical result. Note that the signalsreceived by the UE 207 are also sampled at a rate of 2 samples per chip,with a resolution of 4 bits. The resulting cross-correlation profilesshow one clearly distinguishable peak 301 in the correlation forscrambling code 0, corresponding to the time offset of the signals fromNode B 201. However, the cross-correlation results for the codes 16 and32 do not yield any clear peaks. This is because the signals received bythe UE 207 from Node Bs 202 and 203 are swamped by the relatively strongreception from Node B 201. Were they visible, these peaks should bepositioned to the right of the visible peak 301 by 1 and 2 microsecondsrespectively for the signals from Node Bs 202 and 203 (corresponding to3.8 and 7.6 chips).

The lack of detection of the signals from 202 and 203 means that it isnot possible to compute an E-OTD position fix, since at least threeindependent timings are needed. It has already been described how theuse of idle periods (e.g. the IPDL method) can be used to overcome thisproblem.

The present invention is now illustrated using the same test system. Inthis case, each sampling device 204, 205, 206 records a section of thesignals transmitted by its associated Node B 201, 202, 203 respectively.This section is one symbol in duration and is again sampled at a rate of2 samples per chip, with a resolution of 4 bits. The UE 207 also recordsa 256-chip section of the signals it receives, aligned with the firstsymbol on the CPICH in a particular timeslot, at the same sampling rateand resolution.

At the SMLC 208, the three recordings reported by the three samplingdevices 204, 205, 206 are each cross-correlated in turn with therecording made by the UE 207, and the results are shown in FIG. 4. Thepeaks of the resulting correlation profiles are used to determine therelative levels of the three contributions in the received signal andhence the order in which they are to be subtracted. Once again, thecross-correlation for Node B 201 yields the largest peak 401. Note alsothat, in contrast with FIG. 3, the cross-correlation for Node B 202 alsoyields a clear peak 402. This is because the cross-correlation isperformed using the total signal transmitted by the Node Bs rather thanmerely using the CPICH, which represents a fraction of the totaltransmitted energy in each case.

Having identified the time offset of the signal from Node B 201, therecording of the signal reported by the sampling device 204 is now usedto construct an appropriately scaled, delayed and phase-rotated copy ofthat signal. The results of this process are plotted in FIG. 5. Theupper plot shows the real component of the original signal recorded bythe UE 207 as a solid curve whilst the dotted curve shows the estimatedscaled, delayed and rotated signal. The lower plot shows a similarcomparison of the imaginary parts of received and estimated signals.Note that whilst a duration of 256 chips is actually used in theexample, the time axis in this Figure has been limited to about 50chips. The estimated recordings are subtracted from the total UErecording leaving a residual recording.

The recordings from the sampling devices 205 and 206 are nowcross-correlated with the residual recording giving the results shown inFIG. 6. Note that in this case, following the removal of the signal fromNode B 201, there is a clear correlation peak 601 for the signals fromNode B 203 as well as a peak 602 for Node B 202. These peaks are used toestimate the time offsets of the corresponding signals, givingsufficient independent timing measurements (three in this case) tocompute a position fix.

If the peak 601 corresponding to the signals from Node B 203 is too weakto be resolved, a further iteration could be undertaken in which thesignals from Node B 202 could be subtracted to yield a second residualsignal (FIG. 7). There is a clear correlation peak 701 at a delay ofapproximately 7 chips as expected.

In summary therefore, the problem caused by the relatively high level ofthe signals from Node B 201, which prevents measurement of the timeoffsets for the signals from Node Bs 202 and 203 by the conventionalmethod, is overcome by the iterative approach of the present inventionwhich involves the estimation and subtraction of the strongest remainingsignal so that the next weaker one can be detected.

As explained earlier, the present invention can also be applied to thefixed receivers (LMUs) of a positioning system using conventional E-OTDtechniques. In this case, a fixed receiver is usually co-sited with thetransmitter, but is connected to a separate receiving antenna. The LMU,which needs to support a large dynamic range and display exceptionallygood linear characteristics, receives the signals picked up by itsantenna, creates a section of a representation of the signals asdescribed above, and sends the section to a computing device. A samplingdevice associated with the transmitter provides a contemporaneoussection of the signal transmitted by the transmitter. A calculation isthen carried out in which an estimate of the signal transmitted by thetransmitter and picked up by the receiving antenna is subtracted fromthe representation (to produce a residual representation) in order toreduce its effect on the signals received from the other, more-distant,transmitters of the network. Thus far, the process is exactly asdescribed in the particular example discussed above. The purpose of theLMU, however, is to furnish as many timing offsets between the signalsreceived from all the transmitters as it can. The very strong signalreceived from the local transmitter furnishes an accurate timing forthat transmitter through analysis of the section of the representationsent by the LMU to the computing device, and the residual representationmay then be analysed for the remaining signals from other transmitters.

The analysis of both the representation and the residual representationto find the time offset of a particular component may be carried out asfollows. The pilot code, transmitted on CPICH by a given transmitter, isknown in advance as a binary sequence. This is modulated by passing itthrough, for example, a raised-cosine filter so that it matches asclosely as possible the signal received from the transmitter on CPICH.This reference sequence is then cross-correlated with the section of therepresentation of the received signals, or the section of the residualrepresentation, in order to identify a peak corresponding to the timeoffset of the signal received from the corresponding transmitter withrespect to the reference, as illustrated above in FIG. 3.

1. A method of finding the position or state of motion of a terminal ina communications network having a plurality of transmitters, theterminal having a radio receiver, the method comprising the steps of (a)creating at the terminal a section of a representation of the signalsfrom the transmitters received by the radio receiver; (b) creating afirst section of a representation of the signal transmitted by a firstof said transmitters, and creating a second section of a representationof the signals transmitted by a second of said transmitters, each ofwhich sections overlaps in time with the section created at theterminal; (c) calculating an estimate of the signal received at theterminal from said first transmitter using said first section, andsubtracting said estimate from the section created at the terminal, toproduce a residual representation; (d) performing a calculation usingsaid residual representation and said second section, and estimating thetime offset between them; and (e) calculating the position of theterminal using said time offset.
 2. A method of finding the position orstate of motion of a terminal in a communications network having aplurality of transmitters, the terminal having a radio receiver, themethod comprising the steps of (a) creating at the terminal a section ofa representation of the signals from the transmitters received by theradio receiver; (b) creating a section of a representation of the signaltransmitted by one of said transmitters, which overlaps in time with thesection created at the terminal; (c) calculating an estimate of thesignal received at the terminal from said transmitter using said sectionof the representation of the signal transmitted by said transmitter, andsubtracting said estimate from the section created at the terminal, toproduce a residual representation; (d) performing one or morecalculations using said residual representation and one or more knowncomponents of the signals transmitted by the communications network inorder to estimate the time offset of the respective component; and (e)calculating the position of the terminal using any of said time offsets.3. A method of finding the time offset between at least one of thesignals received from a plurality of the transmitters of acommunications network by a receiver attached to a fixed terminal and areference generated in the fixed terminal, the method comprising thesteps of (a) creating at the fixed terminal a section of arepresentation of the signals from the transmitters received by theradio receiver; (b) creating a section of a representation of the signaltransmitted by one of said transmitters, which section overlaps in timewith the section created at the fixed terminal; (c) calculating anestimate of the signal received at the fixed terminal from saidtransmitter using said section of a representation of the signaltransmitted by said transmitter, and subtracting said estimate from thesection created at the fixed terminal, to produce a residualrepresentation; and (d) performing a calculation using said residualrepresentation and said reference in order to estimate the time offsetbetween the at least one signal and said reference.
 4. A methodaccording to any of claims 1, 2 or 3, wherein the or each section of arepresentation of the signal transmitted by a respective transmitter iscreated at a respective transmitter.
 5. A method according to any ofclaims 1, 2 or 3, wherein the or each section of a representation of thesignal transmitted by a respective transmitter is created in a samplingdevice associated with the respective transmitter.
 6. A method accordingto any of claims 1, 2 or 3, in which the signal representation sectionsare sent to one or more computing devices in which said estimates andtime offsets are calculated.
 7. A method according to claim 6, in whichthe terminal location is calculated in said one or more computingdevices.
 8. A method according to claim 1, in which the time offsetbetween said section of a representation of the signals received by thereceiver and said first section is first calculated, and then used inthe calculation of said estimate.
 9. A method according to any of claims1, 2 or 3, in which the section of the representation of the signalsreceived by the receiver at the terminal is recorded in the terminalbefore being sent to one or more computing devices.
 10. A methodaccording to any of claims 1, 2 or 3, in which the section of therepresentation of the signals received by the receiver at the terminalis transferred in real time to one or more computing devices and arecording or recordings made there.
 11. A method according to claim 6,in which the computing device is in the handset.
 12. A method accordingto claim 6, in which one or more of the computing devices each comprisea processor connected to the network.
 13. A method according to any ofclaims 1, 2 or 3, in which the representation of the signals received bythe receiver is a digitised version of the received signals convertedfirst to baseband in the receiver.
 14. A method according to any ofclaims 1, 2 or 3, in which the representation of the signals transmittedby a transmitter is a digitised version of the transmitted signalsconverted first to baseband.
 15. A method according to any of claims 1,2 or 3, in which, in order to ensure an overlap of the respectivesections, a known component of the transmitted signals is used toindicate the start of sampling.
 16. A method according to any of claims1, 2 or 3, in which the calculations performed includecross-correlations.
 17. A method according to claim 2, in which theknown components of the transmitted signals are the pilot codes.
 18. Acommunications network comprising (a) a computing device or devices; (b)a terminal having a radio receiver attached to the terminal, means forcreating a section of a representation of the signals, received by theradio receiver, from the transmitters of the communications network, andmeans for sending the section to the computing device or devices; (c)sampling devices associated with respective first and second of saidtransmitters for creating respective first and second sections ofrepresentations of the signal transmitted by the respective transmitterwhich overlap in time with the section created at the terminal, and forsending the sections of the representations created at said transmittersto said computing device or devices; the computing device or devicesbeing adapted to perform
 1. a calculation of an estimate of the signalsreceived at the terminal from said first transmitter using said firstsection;
 2. a subtraction of said estimate from the section sent by theterminal, to produce a residual representation;
 3. a calculation usingsaid residual representation and said second section to produce anestimate of the time offset between them; and
 4. a calculation of theposition of the terminal using said time offset.
 19. A communicationsnetwork comprising (a) a computing device or devices; (b) a terminalhaving a radio receiver attached to the terminal, means for creating asection of a representation of the signals, received by the radioreceiver, from the transmitters of the communications network, and meansfor sending the section to a computing device; (c) devices associatedwith the transmitters for creating sections of representations, of thesignal transmitted by the respective transmitter, which overlap in timewith the section created at the terminal, and for sending said sectionsto the computing device or devices; the computing device or devicesbeing adapted to perform
 1. generation of a reference signal; 2.calculation of an estimate of the signal received at the terminal fromsaid transmitter using said section of a representation of the signaltransmitted by the corresponding transmitter;
 3. a subtraction of saidestimate from the section sent by the terminal to produce a residualrepresentation;
 4. one or more calculations using said residualrepresentation and said reference to estimate the time offset between acomponent of the residual representation and said reference; and
 5. acalculation of the position of the terminal using the or any of the timeoffsets.
 20. A communications network according to claim 18 or claim 19,in which the, section of the representation of the signals received bythe receiver at the terminal is recorded in the terminal before beingsent to a computing device.
 21. A communications network according toclaim 18 or claim 19, in which the section of the representation of thesignals received by the receiver at the terminal is transferred in realtime to a computing device and a recording made there.
 22. Acommunications network according to claim 18 or claim 19, in which thesection of the representation of the signal transmitted by a transmitteris obtained from a sampling device associated with the correspondingtransmitter.
 23. A communications network according to claim 18 or claim19, in which the computing device is in a handset.
 24. A communicationsnetwork according to claim 18 or claim 19, in which the computing devicecomprises a processor connected to the network.
 25. A communicationsnetwork according to claim 18 or claim 19, in which the representationreceived by the receiver is a digitised version of the received signalsconverted first to baseband in the receiver.
 26. A communicationsnetwork according to claim 18 or claim 19, in which the representationof the signal transmitted by a transmitter is a digitised version of thetransmitted signal converted first to baseband.
 27. A communicationsnetwork according to claim 18 or claim 19, in which, in order to ensurean overlap of the respective sections, a known component of thetransmitted signals is used to indicate the start of sampling.
 28. Acommunications network according to claim 18 or claim 19, in which thecalculations performed in the computing device includecross-correlations.
 29. A communications network according to claim 19,in which the component of the residual representation is a pilot code.30. A computing device or devices for use in a communications networkcomprising a terminal having a radio receiver attached to the terminal,means for creating a section of a representation of the signals,received by the radio receiver, from the transmitters of thecommunications network, and means for sending the section to thecomputing device or devices; and sampling devices associated withrespective first and second of said transmitters for creating respectivefirst and second sections of representations of the signal transmittedby the respective transmitter which overlap in time with the sectioncreated at the terminal, and for sending the sections of therepresentations created at said transmitters to said computing device ordevices; the computing device or devices being adapted to perform
 1. acalculation of an estimate of the signals received at the terminal fromsaid first transmitter using said first section;
 2. a subtraction ofsaid estimate from the section sent by the terminal, to produce aresidual representation;
 3. a calculation using said residualrepresentation and said second section to produce an estimate of thetime offset between them; and
 4. a calculation of the position of theterminal using said time offset.
 31. A computing device or devices foruse in a communications network comprising a terminal having a radioreceiver attached to the terminal, means for creating a section of arepresentation of the signals, received by the radio receiver, from thetransmitters of the communications network, and means for sending thesection to the computing device or devices; and devices associated withthe transmitters for creating sections of representations, of the signaltransmitted by the respective transmitter, which overlap in time withthe section created at the terminal, and for sending said sections tothe computing device or devices; the computing device or devices beingadapted to perform
 1. a generation of a reference signal;
 2. acalculation of an estimate of the signal received at the terminal fromsaid transmitter using said section of a representation of the signaltransmitted by the corresponding transmitter;
 3. a subtraction of saidestimate from the section sent by the terminal to produce a residualrepresentation;
 4. one or more calculations using said residualrepresentation and said reference to estimate the time offset between acomponent of the residual representation and said reference; and
 5. acalculation of the position of the terminal using the or any of the timeoffsets.
 32. A computer program or programs comprising computer programcode means adapted to perform the steps of the computing device of claim30.
 33. A computer program or programs comprising computer program codemeans adapted to perform the steps of the computing device of claim 31.